1. Field of the Invention
The field of invention relates to compositions, delivery systems, and methods suitable for the enhanced oil recovery process.
2. Description of the Related Art
Surfactants are used in enhanced oil recovery (EOR) processes. The presence of a surfactant at the water/oil interface facilitates oil recovery.
Conventional methods feed surfactant solution into the reservoir directly to maximize the surfactant dose. The active surfactant is readily consumed at the oil-water interface. Surfactants and other chemicals are mixed with water and driven into the reservoir.
Significant amount of surfactant is lost due to adsorption to the rock of the formation before it has a chance to encounter residual oil. Often, surfactant molecules, especially anionic surfactants, adsorb into the rock before it interacts with oil. Adsorption of the surfactant into the formation results in a reduction in the effective amount of surfactant useful for mobilizing the oil. The adsorption problem worsens when anionic surfactants encounter carbonate rock, which acts as a Lewis acid. The extent of surfactant adsorption depends on the type of rock, pore surface area, water salinity and the type of surfactant. In general, roughly one milligram of surfactant adsorbs into one gram of rock. In addition to adsorption, some surfactants precipitate from the saline water with cations from the rock. In either case, some surfactants are lost before they have a chance to solubilize oil.
To overcome the adsorption and precipitation problems, increasing the amount of surfactants in water allows for the extra surfactants to be absorbed by the rock and still deliver remaining surfactants to the oil/water interface. However, the additional surfactant cost is prohibitive. Others use sacrificial chemicals to passivate the rock surface such that surfactant adsorption is mitigated before application.
When a slug of surfactant mixed in a polymer solution is introduced into the reservoir, the surfactant quickly deposits and, depending on the amount used, may be saturated on the oil-water interface. The oil, thus solubilized by the surfactant, can be recovered. Unfortunately, residual oil that lies behind the initial oil-water interface has less a chance of interacting with the surfactant and may remain immobilized.
Overall, the approaches fail to maintain a constant concentration of surfactant over an extended period in the formation. It would be ideal to have a sustained and constant concentration of free surfactant at the water/oil interface to prolong and improve oil recovery while avoiding adsorption into the formation.